The present invention relates generally to refrigeration systems. More particularly, the present invention relates to compressor-driven refrigeration chillers in which at least some lubricant tends to make its way from the system compressor to the system evaporator during the course of chiller operation. With still more particularity, the present invention relates to apparatus and a method by which to return oil from the evaporator to the compressor in a refrigeration chiller using hot compressor oil as the motive force for accomplishing oil return.
The migration of lubricant from the compressor to the evaporator in a compressor-driven refrigeration chiller is an age old problem. A very large number of systems, apparatus, methods and schemes have been used and/or suggested to accomplish the return of such oil from the evaporator of a chiller, where it settles on or into the liquid refrigerant pool found therein, back to the chiller's compressor, where it is needed for lubrication. Many of such systems/schemes employ the use of an eductor which draws oil-rich liquid from the system evaporator using a pressurized fluid sourced from elsewhere within the chiller system as the motive force by which the eductor is powered.
More recently, evaporators of the so-called falling film type have begun to be employed in refrigeration chillers, such evaporators being more efficient in terms of the vaporization process that occurs therein. Falling film evaporators operate such a large majority of the refrigerant that enters the evaporator is vaporized within the evaporator shell before having a chance to pool in liquid form in the bottom thereof. This results in the development of a more concentrated and homogenous oil-rich pool of fluid at the bottom of the evaporator shell, such pool being relatively much shallower than the liquid pools in so-called flooded evaporators where the majority of the tubes in the evaporator's tube bundle are bathed in liquid refrigerant at the top of which an oil-rich mixture is found.
One oil return arrangement employed in refrigeration chillers having falling film evaporators is the one described in U.S. Pat. No. 5,761,914, assigned to the assignee of the present invention and incorporated herein by reference, which teaches the use of a so-called "flush"-type oil return system. Other than eductors/ejectors and flush-type systems, mechanical arrangements of still other and different kinds have been employed to induce or accomplish oil return from the evaporator to the compressor in a refrigeration chiller system. Many such systems are relatively difficult and/or expensive to manufacture and/or control but accomplish oil return nonetheless. Each of such systems brings with it various negative attributes, difficulties, failure modes and expense that detract from their attractiveness with respect to the oil return process.
The need continues to exist for an improved lubricant return system in a refrigeration chiller which efficiently returns lubricant from the system evaporator to the system compressor in a reliable yet simple and inexpensive manner.